Many linear or fixed step size delta modulators experience difficulty in following sudden variations in input signal amplitude. Whenever an abrupt transition from one amplitude to a much different amplitude occurs, it takes a given length of time for the differential encoding process to integrate to a corresponding change in the encoded representation of the signal. This condition is known technically as "slope overload," and arises from the fact that in conventional delta modulators, there are certain practical limitations upon the size of the increments by which the amplitude of the reconstructed waveform is changed from one sample time to the next. If the increment is made small, the modulator responds slowly to steep transitions in the amplitude of the input signal waveform, thereby introducing phase shifts and other distortions into the reconstructed signal waveform. On the other hand, if the increment is made large enough so that the encoding process immediately starts to follow a very steep and prolonged transition of the input signal amplitude, then, with the same large increment, the system becomes unstable and shows a tendency to overshoot and oscillate when the transition peak is reached. Also, an undesirable amount of granular noise may be generated during intervals when the input signal amplitude is constant or only slightly varying. Certain delta modulators, which are called adaptive delta modulators, provide a differential encoding increment of adjustable size which is small at times when the system is idling and larger at times when the signal amplitude is varying rapidly. In some prior art delta modulators there is a fixed threshold value to which the dalta increments are compared for determining the amount the new prediction signal is changed, while in other delta modulators the threshold value is a state dependent value, as is a delta increment. In the latter type delta modulator, a new threshold value is chosen at each sample time, as is the delta increment. One such delta modulator is disclosed in U.S. Pat. No. 3,628,148 issued in the name of Steven J. Brolin. In the Brolin patent the number of available delta increments and the number of available threshold values are determined by the number of stages in the input storage register in which the respective code symbols are stored. To increase the number of available delta increments and available threshold values, the number of stages in the storage register must be increased and accordingly the size of the delta increment table and threshold table must be increased. In such a system, to have available an optimal number of threshold values and delta increments, the number of circuits and logic elements increase accordingly, resulting in an increase in the cost and size of the delta modulator.
According to the present invention, a delta modulator is disclosed which also utilizes a table of threshold values as well as a table of delta increments, which respond to stored code symbols for selecting threshold values and delta increments. An optimal number of delta increments and threshold values are available, not by utilizing more storage stages and larger tables, but rather by using a modifying network which consists of an integer arithmetic digital gain logic circuit which also responds to the stored code symbols for providing a modifying signal which modifies the selected delta increment and the selected threshold value, thereby in effect increasing the effective size of the respective tables, while minimizing the increase in circuits and logic elements and accordingly the attendant cost of such elements.